Nano-filtration, also known as low-pressure reverse osmosis, is characterized by its high retention rate for divalent or multivalent ions and small molecules with molecular weight cut-off in the range of 200 to 1000 g·mol−1, while its retention rate for monovalent ions is relatively low. With the development of membrane separation technology, the application of nano-filtration has been extended to industrial fields such as drinking water purification, wastewater treatment, grading and concentration of pharmaceutical products, solvent recovery and so on.
At present, the membrane-forming materials commonly used such as PVDF (polyvinylidene fluoride), PSF (polysulfone) or PES (polyether sulfone) are all linear polymers, in which the spacing between molecular chains is large, resulting in the poor desalination effect. Although a three-dimensional structure can be formed by crosslinking, the hydrophilicity of the material has great influence on the flux and anti-pollution property of the membrane. After the functional groups of the hydrophilic crosslinking monomers have been cross-linked, the functional groups are consumed, thereby reducing the hydrophilicity, flux and anti-pollution property of the membrane. In addition, microorganisms and bacteria in water will deposit and reproduce on the nano-filtration membrane during its use, and form a biofilm which results in pollution of the nano-filtration membrane, causing a sharp decrease in the flux of the membrane. Therefore, in order to solve one of the above problems, it is particularly necessary to develop a method for preparing a hollow fiber nano-filtration membrane which has excellent mechanical property, high membrane flux, and high anti-pollution, anti-bacterial properties.